Turbochargers for gasoline and diesel internal combustion engines are devices known in the art that are used for pressurizing or boosting the intake air stream, routed to a combustion chamber of the engine, by using the heat and volumetric flow of exhaust gas exiting the engine. Specifically, the exhaust gas exiting the engine is routed into a turbine housing of a turbocharger in a manner that causes an exhaust gas-driven turbine to spin within the housing. The exhaust gas-driven turbine is mounted onto one end of a shaft that is common to a radial air compressor mounted onto an opposite end of that shaft. Thus, rotary action of the turbine also causes the air compressor to spin within a compressor housing of the turbocharger that is separate from the exhaust housing. The spinning action of the air compressor causes intake air to enter the compressor housing and be pressurized, or boosted, a desired amount before it is mixed with fuel and combusted within the engine combustion chamber.
Because the rotary action of the turbine is dependent upon the heat and volumetric flow of exhaust gas exiting the engine, turbochargers are often of reduced effectiveness when the engine to which they are coupled is run at a low speed.
The reduced effectiveness is often labeled turbo-lag. In order to overcome turbo-lag when the heat and volumetric flow of exhaust gas is low, an electric motor is known for rotating the shaft and inducing the compressor to spin.
Electrically assisted turbochargers are, however, especially susceptible to entering compressor surge regimes because such electric control of the compressor can enable the compressor to function in a manner that is relatively independent of engine operating conditions. Generally speaking, compressor surge is a turbocharger condition whereby pressurized air that is created by the compressor meets an internal system resistance, oftentimes causing the pressurized air to be forced backwards through the turbocharger. Surge can occur from different turbocharger operating conditions, and is known to occur during engine operating conditions of deceleration. Compressor surge is generally an undesirable condition that can cause several problems from noise to component failure, which can be detrimental to turbocharger life and performance. For these reasons, compressor surge is undesirable.
Therefore, it is desirable to provide a system for use with an electrically assisted turbocharger that enables control of the electric motor to protect the compressor against entering a compressor surge regime, thereby prolonging turbocharger service life and desired turbocharger performance.